The present invention relates to an optical line terminal and an optical network terminal in an optical access system for providing communication between a subscriber's home and a central office of a carrier.
Of public communication networks for transferring data such as audio and video, there are used telephone subscriber network, ADSL, and other networks to implement an access network in which users are accommodated in a central office. Recently introduction of an optical access system has been started.
A known example of the optical access system is PON (Passive Optical Network) as a mode of 1 to n connection between the central office side and the subscriber side. The PON provides data communication between an OLT (Optical Line Terminal) located in a central office and plural ONTs (Optical Network Terminals) located in subscribers homes, using a shared bandwidth in such a way that each optical wavelength is assigned for ascending and descending. In the case of signals on the descending side from the OLT to the ONTs, an optical signal is split into signals halfway through in a splitter. The ONT side extracts only a signal addressed to the own terminal. Thus communication is established therebetween. In the case of signals on the ascending side, the OLT notifies the ONTs of transmission time timing. The ONTs transmit signals to the OLT in accordance with the timing. In this way plural ONTs communicate with the OLT by sharing a single wavelength.
Known examples of such an optical access system are B-PON (Broadband PON), A-PON (ATM PON), GE-PON (Giga-bit Ether PON), and G-PON (Gigabit-capable PON). Particularly G-PON attracts attention for the following reasons. That is, G-PON is the fastest with a maximum ascending speed of 1.25 Gbits/s and a maximum descending speed of 2.5 Gbits/s, serving plural protocols by adopting a native GEM (Gigabit-capable Encapsulation Method/G-PON Encapsulation Method) that provides support for ATM, Ethernet, and WDM protocols.
As described above since the splitter exists between the OLT and the ONT, both the descending and ascending signals are attenuated. Thus the application of encoding/decoding technology of forward error correction code (FEC) is being investigated also for the PON. Among block codes having a mathematically consistent system, the most commonly used forward error correction code is a code that is called a systematic code due to its transparency of information. The systematic code partitions a sequential signal series into a certain number of blocks to perform encoding for each block. The systematic code has a feature of only adding check bits to a predetermined space area of a signal, without operating the original signal information. Examples of the block code are Hamming code, BCH code (Bose-Chaudhuri-Hocquenghem code), and Reed-Solomon code, which have been used for long time. Hereinafter encoding and decoding of the forward error correction code is simply referred to as encoding and decoding.
In U.S. Pat. No. 7,024,616 B2, there is described a method for configuring an error correction code, and transmitting equipment. The error correction code has a higher gain while maintaining an original transmission distance when an optical signal transmitted through a single-core fiber is wavelength multiplexed even if the degree of time division multiplexing for the optical signal is approximately doubled, being capable of increasing the repeater spacing of the optical signal by a factor of four, and having a high degree of interconnectivity with the existing transmission network to which an eight-error-correcting Reed-Solomon code defined in ITU-T Recommendation G.975 is introduced.